An image sensor or imaging sensor is a sensor that can detect a spatial intensity distribution of a radiation. An image sensor usually represents the detected image by electrical signals. Image sensors based on semiconductor devices may be classified into several types, including semiconductor charge-coupled devices (CCD), complementary metal-oxide-semiconductor (CMOS), N-type metal-oxide-semiconductor (NMOS). A CMOS image sensor is a type of active pixel sensor made using the CMOS semiconductor process. Light incident on a pixel in the CMOS image sensor is converted into an electric voltage. The electric voltage is digitized into a discrete value that represents the intensity of the light incident on that pixel. An active-pixel sensor (APS) is an image sensor that includes pixels with a photodetector and an active amplifier. A CCD image sensor includes a capacitor in a pixel. When light incidents on the pixel, the light generates electrical charges and the charges are stored on the capacitor. The stored charges are converted to an electric voltage and the electrical voltage is digitized into a discrete value that represents the intensity of the light incident on that pixel.